It is known that an ideal injection system is perfectly linear, i.e. the flow rate at which the petrol is injected (given an identical pressure difference across the injector) is directly proportional to the injection time; thus, for an ideal injection system, the characteristic curve for injection time/flow rate of injected petrol is a straight line. A real injection system differs from the ideal injection system in that it tends to have zones of non-linear operation, i.e. zones of operation in which the characteristic curve for injection time/flow rate of injected petrol deviates from the ideal straight line; in particular, real injection systems have optimum linearity at long injection times, while they tend to move away from the ideal characteristics as injection times shorten.
A parameter known as dynamic range is used to characterise the degree of compliance with ideal linearity, said parameter expressing the ratio between the maximum injectable flow rate and the minimum injectable flow rate while keeping the deviation from the ideal straight line within a predetermined margin.
Known injection systems provide for the control of each electromagnetic injector by applying a relatively high voltage (approx. 70 volts) to the associated control coil for a first time interval (typically of less than 0.1 msec) in such a manner as to bring about a rapid rise in the current passing through the injector until a premagnetisation value is reached (indicatively of around 2 amps), which is maintained for a premagnetisation time of approx. 0.2 msec by cyclic switching (known as “chopping” in the art) of the voltage between zero and the maximum value; a relatively high voltage (approx. 70 volts) is then applied again for a second time interval (typically of less than 0.3 msec) during which the current passing through the injector reaches a peak value (approx. 10–12 amperes). Once the current passing through the injector has reached the peak value, the voltage across the injector is set to zero or even inverted (i.e. by applying −70 volts) so that the current is rapidly adjusted to a holding value (approx. 2 amperes), which is maintained until completion of the planned injection time by cyclic switching (known as “chopping” in the art) of the voltage between zero and a holding value (typically the battery voltage of 12 volts). At the end of the planned injection time, the current passing through the injector is set to zero by applying an inverse voltage to the elevated value (approx. −70 volts). Experimental testing has revealed that the injector opens shortly after reaching the peak current and closes after the current passing through the injector itself is set to zero. The delay between the current starting to flow through the injector itself and the injector actually opening and the delay between the current ceasing to flow through the injector itself and the injector actually closing have effects on the quantity of fuel injected, which tend to cancel each other out and that, being at a first approximation effects that are independent of injection time, have no impact upon the linearity of the injection time/flow rate of injected petrol relationship, but simply shift (“offset” in the art) this constant relationship relative to zero.
Various experimental tests have demonstrated that currently available electromagnetic petrol injectors controlled in accordance with the above-described method and operating at a pressure of 100 bar have dynamic range values lower than those obtained from the low-pressure (3–5 bar) injectors for indirect injection that are currently in production but, in order to ensure optimum functioning of a direct injection petrol engine, it is necessary to use an injection system that has a dynamic range of at least 12.
Currently available petrol injectors controlled in accordance with the above-described method have a relatively mediocre dynamic range in that, within a critical time interval between opening of the injector and reaching the holding value, the current passing through the injector varies very rapidly and a closure command issued within the critical interval can result in very different actual closure times depending upon the value for current prevailing at the moment at which the closure command is applied.
In an attempt to overcome the disadvantages described above, it has been proposed to use an electromagnetic injector provided with two coils and two respective independent control circuits; a main coil is used to open the injector, while an auxiliary coil is used only during the closure phase to accelerate closure of the injector. However, this solution is distinctly costly and complex since it provides for the addition of the auxiliary coil and the associated control circuit.
EP1201898 discloses a device for controlling fuel injection and comprising fuel pressure regulator means for adjusting fuel pressure of the fuel to be injected, injector drive means including an electromagnetic coil for opening the valve body of the injector, and injection control means for controlling the fuel regulator means and the injector drive means depending upon the operating conditions; the injection control means includes an injection timer for setting the exciting time for controlling the driving time for opening the valve body by controlling the exciting current and the exciting time for the electromagnetic coil, an over-excitation timer for feeding an over-exciting current, and an over-exciting period control unit for variably setting the initial count value of the over-excitation timer depending upon the fuel pressure. The over-exciting period is variably set to be a minimum required limit that increases with an increase in the fuel pressure.
EP0889223 discloses a method for detecting the switching time of an electrovalve; the method involves controlling the current flowing through the solenoid valve coil. In a first phase, the trip current is controlled at a defined value. During a second phase, a retaining current is controlled at a defined value. During a detection phase, when it is likely that the switching point will occur, the current is controlled at a detection value, which is smaller than the trip current and larger than the retaining current. The detection value is selected such that no saturation occurs. When the solenoid valve is used in a fuel injection system, the injection is divided into a pre-injection and a main injection. During the pre-injection, the current is controlled during the detection phase at the detection value. When the solenoid valve is used in a fuel injection system, the injection is divided into a pre-injection and a main injection. During the pre-injection, the current is controlled during the detection phase at the detection value.
EP0704096 discloses a system and method for operating high speed solenoid actuated devices such as electromagnetically operated high pressure fuel injectors requiring an initial high power boost to start the movement of an armature followed by a medium power boost to continue the movement of the armature to its end position and a low power control to hold the armature at its end position so that when the power is removed, the armature returns to its rest or beginning position. The system here details the logic and control necessary to provide six stages of power control, including both voltage and current control, to accomplish high speed operation both in moving the armature from its beginning to end position but also to return the armature from its end to its beginning position.